Volcanism, Impacts, and Mass Extinctions: Causes and Effects is a record of a 2014 symposium edited by Gerta Keller and Andrew Kerr. It's mostly earth scientists who favor vulcanism as the main driver of mass extinctions. Yet the astronomers (whose papers are clustered at the end--eek, an orbital ghetto!) have a lot of surprises too. Skim the geologists (some redundancy), but hang in there for the astronomers.

Each group admits openly that they haven't fully SEEN the other's evidence, let alone fairly evaluated it. Just too specialized! The details get obscure (these are symposium papers, not pop science articles) but the gist is easy: both sides are right. Poison Gaia OR whack her on the head--either can kill. The great question in my mind now is how vulcanism and impacts interact--and on that cross-disciplinary topic, you're as much of an expert as these specialists! I'd urge interested readers to dive in--primary sources, on a topic like this, really are better.

Huge basaltic flows ("traps") are scattered all over Earth's surface and through time; and the correlation between big ones and extinction events is near-perfect. The lone exception is a massive glaciation c.430 million years ago. What does fit all of them? Big fast temperature swings--up or down, no matter. It's change, not causes, levels or directions, that thins out life. That has implications for our Ice Ages--the relentless barrage of temperature swings matches the pattern for past mass extinctions even without tossing humans in the mix.

The dinosaurs? The Deccan Traps fit perfectly for the that extinction; three flows, with the middle one, 70-80% of the total, right at the boundary; the final after-flow may explain life's unusually slow recovery. The middle eruption was the longest known lava flow in Earth's history, a good 1500 km/950 mi, right across India from near Mumbai to the east coast and into the sea (near the antipode of Chicxulub, and I refuse to accept that's coincidence.) Gerta Keller et al make a strong case that the sulfur and CO2 from these flows destabilized climate and stressed life tremendously in the Old World and probably worldwide.

The weaker case of impacts generally (as opposed to the dramatic case of Who Killed the Dinos, where evidence is pretty strong--I'm not convinced by Keller's dismissal of New World tsunami evidence) is partly because they're less known and mapped; from cratering rates on the Moon, it's clear that erosion, ice and deep water have hidden most of Earth's impact scars. Bill Napier, in one paper, estimates only one-seventh of the craters that have to exist have been found (in contrast, basaltic flows are hard to miss). Nor are craters widely known even when found! For example; I build whimsical alternate Earths that require me to know the sea floor pretty well, but I didn't learn till this year of the Eltanin impact (c.30 km wide, off southern Chile; just a couple of million years ago); and I'm not alone. It's missing from Napier's list of large craters (3km+) (p.389), though it fits his criteria and adds to his most recent impact-wave. A few months later I was idly leafing through a picture book on Australia and came across Wolf Creek Crater, smaller, but also fresh, within two million years... In short: Earth's craters get neglected. Neither quite geology nor astronomy! Remember how the evidence for Chicxulub, biggest of all, sat in an oil company's files for decades. Specialization limbo!

Do impacts come in waves? Here--see for yourself. Blue lines stand for about five million years, back to 220 million years ago; from 220-450 MYA, they represent ten million each. Remember, each proposed cluster is really just one blue line tall! Just want you to get a sense how sporadic cratering seems.

Is Bill Napier cherry-picking his craters? In hand-transcribing and tweaking his chart, I omitted, for simplicity, his estimates of uncertainty; he only listed ones with ages definite enough to let any possible clustering show--many aren't even dated to within ten million years and are useless for this purpose. I've compared Napier's to his source databases online and concluded he's playing fair. These assault waves look real.

Such clustered impacts imply these aren't asteroids--they may be at most 5% of impacts over deep time. Our current comet-free rate lulls us, but comets are the real risk--and explain the clustering. Huge objects (up to 250 km!) from the Kuiper belt occasionally get perturbed and swing inside the outgas line, destabilize, and break up over a thousand to a few hundred thousand years; even if we're lucky enough to avoid a Chicxulub (or two), we get waves of fragments of all sizes--and firestorms, tsunamis, toxins, ozone depletion, dust and gouged crust and splatted seawater, all destabilizing climate.

So the risk is about 20 times higher than asteroids imply--and cometary fragments can be much bigger, and hit faster, so it's scale not just frequency we've underestimated.

And we're in such an impact wave right now! Eltanin Crater fits into a pattern; the Ice Ages have been one of those high-impact periods. Lack of a visible comet could mean our current shower's over except for the rubble, or that the period of the mother comet is just too long for us to have seen it historically.

Now I'm suspicious of the pat explanations for our Ice Age--both its mere existence after ages ice-free, and its interglacial cycle. Axes and precession and shifts in Atlantic currents? Maybe, maybe not. Weren't they present all along? What if periodic comet showers really drive our age of temperature instability? We haven't considered the possibility.

I've always assumed Ice Age megafauna extinctions were anthropogenic--hunting and burning--but this symposium makes me more skeptical. You see exactly this pattern around the KT boundary--big temperature swings and megafauna extinction... without spearheads or smokestacks! Our ecological punch is obvious, but looks like just one more vicious body-blow to a life-web already pummeled by dozens of Ice Age temperature swings.

Not much comfort for Republicans, though! Even if the Sixth Extinction isn't entirely our fault, it's happening and we face its dangers. Stabilizing temperature looks vital.

No one in the symposium mentions this, but it implies that what we see as Nature--life "undisturbed"--is really just a shell-shocked shadow of Earth's real potential. Regions that now seem "naturally" barren can flourish; they did in the recent past. We need to correct our view of ecological stewardship from nostalgic restoration of Interglacial life just before horrible us. That life was already devastated. We can do way better than that.

My pet theory that shockwaves from the Chicxulub impact converged on the far side of the world, causing or massively worsening the Deccan Traps eruption, does get mentioned in a couple of the astronomers' papers, but repeatedly dismissed with a reference to Meschede's 2011 paper "Antipodal Focusing of Seismic Waves"... where he concluded Earth's irregularity caused scattered minifocuses not a single big one; enough to crack the crust opposite Chicxulub, but not enough to cause vulcanism in quiet crust.

Nice, quiet crust like the Deccan Traps? Whaaat? This was already the most geologically violent area in the world! Meschede's summary (all I could read on the net) concludes the Deccan Traps were too far away--not perfectly aligned at the antipode. Really? The maps in the conference all confirm my own estimate they were just 1500-2000 km west of the antipode--near enough for quakes way beyond anything we're ever seen. And the area that the longest flow in history spread into (today's Bay of Bengal) headed straight for the antipode. Did it actually flow west to east, or did a line of vents rip open between the existing vents near present-day Mumbai and the antipode? To me that seems a likelier scenario.

My pet theory is naïve. But what's the alternative? The biggest impact ever known happens JUST when the longest lava flow ever known happens at the antipode! With two record-holding rarities, "extraordinary claims need extraordinary proof" applies to the coincidence camp, not the linkage camp. It's like arguing that since people do sneeze and get headaches now and then without the help of bullets, therefore a bullethole and a gaping exit wound are just coincidence. Sorry, no.

Impacts may provoke catastrophic vulcanism; all the more imperative we understand the linkage or firmly disprove it--and in the meantime, do all we can to protect Earth from temperature swings--from any cause.

Lead pipe in the ballroom, gun in the parlor. You're dead either way.

Conclusions?

1. We underestimate vulcanism; it causes mass extinctions. So what causes catastrophic eruptions? I theorize they're at least sometimes provoked by large impacts. If not, why such huge outbreaks followed by long (and irregular) periods with none? Whatever the answer, knowing what causes catastrophes that repeatedly wiped out nearly all life seems way more important than what killed off one group of (admittedly photogenic) creatures in the last big extinction. Disprove my pet theory and cook one up with predictive power!
2. We WAY underestimate impact risk too--by a factor of 20? Worse, we're mapping the wrong objects. Asteroids are small, slow and predictable. Sittin' ducks in the shooting gallery. The big hazard is cometary--they're big (size nearly unlimited), swing in fast (basically falling from infinity) and inherently unpredictable (basically steam-powered rockets!) It'll take a MUCH more robust warning system and intervention system to protect Earth adequately. That will take time--and should reshape our space programs.
3. What can we do now? With current technology?
   A: Drill known craters more, and date them accurately! Find Earth's buried, overlooked craters; how old are they? We need to know if clustering is an artifact of our limited sample! The answer affects our survival strategies. And if clustering is real, does it correlate with major eruptions? That too is vital to know (and, if yes, is Nobel Prize material).
   B: Map Kuiper Belt objects. This will take serious scope time. In particular, Brown & Katygin's proposed eccentric Ninth Planet needs to be proven or shot down--at 1-10 Earth masses, "Planet Ix" would perturb comets and affect bombardment here in the Inner System (and if it exists, is also Nobel Prize material.)

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